1. Field of the Invention
The present invention relates to a rear attachment lens, which is detachably mounted to the image side of an imaging lens (main lens system) used in a digital still camera, a video camera, a camera for broadcasting, etc., and which changes the focal length of an entire system so as to become longer than that of the main lens system.
2. Description of the Related Art
Hitherto, a rear attachment lens that can be detachably mounted to the image side (image plane side) of a main lens system has been proposed. A rear attachment lens is a type of lens extension that can be removably attached to another lens, e.g., a main imaging lens (imaging optical system), and which changes the focal length of an entire system so as to become longer than that of the main lens system. Examples of rear attachment lenses are discussed in U.S. Pat. No. 6,124,981 and Japanese Patent Application Laid-Open No. 2004-226648.
In general, as the magnification of a rear attachment lens increases, the residual aberration of a main lens is enlarged and image quality is reduced. For example, when the magnification increases twice, transverse aberrations such as coma aberration and chromatic aberration of magnification (transverse chromatic aberration) are simply enlarged twice and image quality is reduced. Also, longitudinal aberrations such as spherical aberration, curvature of the field, and axial (longitudinal) chromatic aberration are enlarged by the square of the magnification, i.e., four times. In the case of the rear attachment lens, however, because the F-number of the main lens is also enlarged twice, aberration per unit focal depth is eventually enlarged twice and image quality is reduced. In a telephoto lens employing a rear attachment lens in many cases, particularly, chromatic aberration is enlarged as the focal length increases. Therefore, when the rear attachment lens is mounted, the enlarged chromatic aberration of magnification among various chromatic aberrations becomes a main cause degrading the image quality.
U.S. Pat. No. 6,124,981 and Japanese Patent Application Laid-Open No. 2004-226648 each disclose a rear attachment lens for changing the focal length of the main lens system to the side where the focal length becomes longer, when attachment lens is mounted to the image side of the main lens system. More specifically, U.S. Pat. No. 6,124,981 discloses the rear attachment lens, which is mounted between the main lens system and a camera, and which increases the focal length of the main lens system by about 1.4 to 2.8 times. In the U.S. Pat. No. 6,124,981, the chromatic aberration of magnification is satisfactorily corrected by using a diffractive optical element. Japanese Patent Application Laid-Open No. 2004-226648 discloses the rear attachment lens, which is mounted between the main lens system and a camera, and which increases the focal length of the main lens system by about 1.7 times. In Japanese Patent Application Laid-Open No. 2004-226648, various aberrations are satisfactorily corrected by using a cemented lens made of a lens material having a proper refractive index.
With the relentless progress toward a larger number of pixels and higher image quality, newer image pickup apparatuses (e.g., digital cameras) are strongly demanded to generate high quality images with minimal chromatic aberration even when the rear attachment lens is mounted to a main lens system used in the digital camera.